Oscillators can be broadly classified into crystal oscillators and internal oscillators. A crystal oscillator can have a very stable frequency with precision as high as 1 or 2 parts per million (ppm). On the other hand, internal oscillators, which are built entirely on the silicon chip or with one or few external components (e.g., resistor, capacitors, inductors, etc.), cannot provide the same level of precision. With an internal oscillator, the best precision that can currently be achieved is in the range of 0.5-1.0%; greater precision is desirable.
To achieve high precision in an oscillator, a precision component is necessary, either inside or outside the correction loop. This precision component can be achieved using one or more external components, such as an off-chip resistor-capacitor combination, an internal component with trim, or a combination of these components. Trimming is an operation on a component to obtain a specific accuracy from that component. This operation can utilize a laser to physically burn off portions of a component or the design can provide multiple switches, which can be set after the manufacturing process to provide the desired result. It is nearly impossible and completely impractical to trim on-chip elements, such as resistors or capacitors, to very high accuracy, i.e., in the range of 0.1%. In an example of this difficulty, a 20K resistor with +−15% process variation will need 300 steps with 20Ω each. It is nearly impossible to realize a practical switch smaller than 20Ω by itself. If we cannot achieve high (˜0.1%) accuracy in any internal component, we cannot build an oscillator with high precision using completely on-chip components.
Existing high precision solutions require precision analog modules, such as a very low offset comparator/amplifier, accurate switched capacitor sampling, or high quality switches. Additionally, these high precision solutions require a process trimmed precision analog RC. We have not previously had any method of trimming an internal Resistor-Capacitor (RC) time constant to 0.1%; therefore, we are not able to build a completely on-chip precision oscillator of that level of accuracy. Additionally, non-idealities like switch ON resistance, rise/fall time, incomplete settling, and parasitic coupling limit the accuracy beyond approximately 0.5% in reasonable practical implementation. In addition, for a chip containing multiple oscillators, every oscillator on the chip needs to be trimmed and tuned separately, which leads to high test costs. Further, when a high precision oscillator is tuned to one frequency, it is difficult to later tune the same oscillator to some other frequency with similar high accuracy. In some cases such retuning may be possible over a short range, but is difficult over larger frequency range.